This invention relates generally to RF/ID ("Radio Frequency IDentification") integrated circuits, transponders, and systems. More particularly, the present invention relates to a method and circuit for controlling the level of the RF signals within an RF/ID transponder.
A prior art RF/ID transponder and reader system 10 is shown in FIG. 1, which consists of an RF/ID reader 12 and RF/ID transponder 14. The RF/ID reader 12 is typically a stationary or handheld device that transmits a signal 18 to an RF/ID transponder 14 and receives and interprets a signal 16 transmitted from the RF/ID transponder 14 back to the RF/ID reader 12. The RF/ID reader 12 generates an electrical field and modulates this field to communicate with RF/ID transponder 14. The RF/ID transponder 14 is typically a personal plastic card approximately the size of a credit card, but can also have different sizes and be used in different applications such as a luggage tag, key fob, or the like. RF/ID transponder 14 includes an antenna 20 attached to an RF/ID integrated circuit 22. The antenna 20 is typically separate from the RF/ID integrated circuit 22, but housed with the RF/ID transponder 14. Typically, a single antenna 20 is used for both transmitting data, receiving data and the generation of DC power to the integrated circuit 22. For power generation, current is induced in antenna 20 when it is placed in an electrical field that has a frequency closely matched to the antenna. This current creates a corresponding AC voltage that is rectified into a DC signal with the DC current stored in a storage capacitor (not shown in FIG. 1). The physical distance between the RF/ID reader 12 and RF/ID transponder 14 directly affects the magnitude of the generated AC signal used to power the transponder.
The RF/ID integrated circuit 22 is a single integrated circuit implemented in silicon that allows data to be read and written via modulation of an electrical field consisting of signals 16 and 18. The integrated circuit includes three basic circuit sections. An analog driver section attaches to the external antenna 20. The analog driver section is used to develop a DC voltage to power the integrated circuit 22, to modulate the electrical field for transmitting and receiving data, and to detect modulated receive data. A nonvolatile memory portion of the RF/ID integrated circuit is used to store data. Data can be read from as well as written to integrated circuit 22. Typically, the memory is nonvolatile so that information written to integrated circuit 22 is retained even if the RF/ID transponder 14 is removed from the electrical field emanating from the reader 12. A third section of RF/ID integrated circuit 22 consists of digital logic. The logic section controls the actual behavior of integrated circuit 22 and interfaces the memory section to the analog driver section.
For receiving data from the RF/ID reader 12 to the RF/ID transponder 14, the analog section of integrated circuit 22 uses the same AC voltage that is generated by antenna 20 for power generation. When frequency shift keying ("FSK") is used as a modulation technique, integrated circuit 22 measures the frequency of the generated AC voltage and generates either a logic zero or a logic one data state in the logic portion of the chip depending upon the frequency of the AC signal. The frequency and "Q" of antenna 20 is ideally matched with the modulation frequency of the received data, otherwise the maximum distance of operation between the reader 12 and integrated circuit 14 will be less than optimal.
For transmitting data from the RF/ID transponder 14 to the RF/ID reader 12, the logic portion of the RF/ID integrated circuit 22 creates a digital stream of data bits from the memory area of the integrated circuit. This stream is typically encoded using Miller or MFM encoding. This encoded digital data stream is sent to the analog section of the integrated circuit 22 for modulation and transmission. To transmit data from integrated circuit 22 to the reader 12, current is supplied to antenna 20, which causes an electrical field to be created. This field is then sensed by the RF/ID reader 12 and demodulated/decoded into a digital stream of data. Again, the frequency and "Q" of antenna 20 and the level of current pumped into the antenna determine the magnitude of the signal transmitted from RF/ID transponder 14 to the RF/ID reader 12, which ultimately determines the maximum operational distance.
One of the problems with the scheme of receiving and transmitting data with an RF/ID integrated as described with reference to FIG. 1 is that a proper balance between the receive and transmit signals must be achieved in antenna 20. An incoming carrier signal 18 is the signal that is supplied by the RF/ID reader 12. This signal generates power to the RF/ID integrated circuit 22 and is modulated between two different frequencies when data is written to the integrated circuit. The signal to the RF/ID integrated circuit 22 ("IC") is the signal (current or voltage) that flows within or appears across antenna 20. The signal within antenna 20 will be identical to the incoming carrier signal 16 as long as the RF/ID transponder 14 is not sending data back to the RF/ID reader 12. However, the signal within antenna 20 is modified whenever the RF/ID transponder 14 transmits data back to the RF/ID reader 12 by an outgoing signal 16. This modification of the signal in antenna 20 is a result of the same antenna 20 being used for both transmitting and receiving data. As is apparent from inspection of FIG. 1, the signal in antenna 20 is the sum of the incoming carrier signal 18 and the outgoing signal 16 transmitted from the RF/ID transponder 14. Consequently, the signal 16 transmitted from the RF/ID transponder 14 should be set to the correct signal amplitude to satisfy both of the following boundary conditions:
1. The maximum level of signal 16 should be set to less than a predetermined maximum level to prevent distortion of the input signal in antenna 20 and power level; and
2. The minimum level of signal 16 should be set to greater than a predetermined minimum level so that the RF/ID reader 12 can receive data from the RF/ID transponder at some acceptable operating distance.
Turning now to FIG. 4, this waveform diagram shows that when the signal 16 from the RF/ID transponder 14 is minimized, the signal 20' in antenna 20 has little distortion. This is a very good situation from the standpoint of powering up the RF/ID integrated circuit 22 and receiving data from the RF/ID reader 12. However, the situation illustrated in FIG. 4 greatly reduces the physical distance allowed between the RF/ID reader 12 and the RF/ID transponder 14 since signal 16 has a small amplitude and therefore generates only a weak electrical field.
Turning now to FIG. 5, the opposite condition is illustrated. The amplitude of signal 16 transmitted from RF/ID transponder 14 has been set to a high level. Consequently, the amplitude of signal 20' in antenna 20 has become distorted. When this happens, the RF/ID integrated circuit 22 has difficulty in receiving adequate power since the power is a direct result of the area under the signal 20' waveform, i.e. low alternate peaks in the 20' signal waveform will produce drops in the integrated circuit 22 power. In addition, when the amplitude of signal 20' is lower than the voltage thresholds of the analog circuitry on the RF/ID integrated circuit 22, the transitions of signal 20' will be lost. The loss of transitions will result in multiple undesirable consequences up to and including total loss of communication with RF/ID transponder 14.
Turning now to FIG. 6, an optimum setting for the amplitude of signal 16 is achieved. The maximum level of signal 16 is set less than the predetermined maximum level that causes distortion in signal 20', while achieving more than a minimum signal strength so that an acceptable operating distance between the RF/ID reader 12 and the RF/ID transponder 14 can be obtained.
Turning now to FIG. 2, a prior art RF/ID transponder 14 is shown that is modified in order to achieving the acceptable waveform setting shown in FIG. 6. The component parts of the RF/ID transponder 14 are shown in greater detail in FIG. 2. RF/ID transponder 14 includes an antenna 20 and an RF/ID integrated circuit 22. The circuitry fabricated on the RF/ID IC 22 includes the analog driver and digital interface section 24, digital logic section 26, and nonvolatile memory section 28. The analog driver section 24 is shown to have a differential output for driving the antenna nodes 34 and 36, respectively designated "acplus" and "acminus". Also coupled to antenna nodes 34 and 36 is a power generation block 33, which includes circuitry for converting the waveforms on this node into the VDD and GND power supply voltages used to power integrated circuit 22.
Turning momentarily to FIG. 3, the actual acplus waveform 34' and acminus waveform 36' are shown. The MFM encoding is used to change the phase of the waveforms depending on the data state that is being transmitted. For example, between time points t0 and t1, the phase of the acplus and acminus waveforms are interpreted by the RF/ID reader as a logic one. Between time points t2 and t3 the phases of the acplus and acminus waveforms have shifted by 180.degree. and are interpreted by the RF/ID reader as a logic zero.
Turning back to FIG. 2, note that the RF/ID transponder 14 includes a set of output resistors 30 and 32 that are placed in series with the differential output of the analog driver section 24. The output resistors 30 and 32 are used to achieve the proper balance between the incoming carrier signal 16, the signal 20' in the antenna, and the outgoing signal 18. The resistance values to achieve a proper signal balance according to conditions (1) and (2) given above, several factors should be taken into account. Among these factors are principally the strength of the incoming carrier signal 16 and the desired operating distance between the reader 12 and the transponder 14.
Once the various design factors have been identified, an optimum resistance value for resistors 30 and 32 can be selected. Once the resistance value has been determined, the resistors are simply built into RF/ID transponder 14 as discrete resistances or fabricated within integrated circuit 22. The problem with this approach is that the RF/ID transponder is now permanently tied to a specific application regarding reader signal strength and operating distance. Alternatively, the resistors can be made mask programmable on integrated circuit 22, which allows for a range of operating distances to be accommodated. The problem with this approach is that metal mask programming increases cost and silicon area, as well as time to fabricate a new set of masks. Even though a range of operating conditions can be accommodated by a set of mask programmed RF/ID transponders, each individual RF/ID transponder is still limited to its particular operating conditions.
What is desired is an RF/ID transponder that can be easily programmed to respond to a wide range of RF/ID system operating conditions, without using metal mask programming.